There are no charges for Camp Carnival, although I'm sure they'd appreciate a tip at the end of the cruise.
My wife and daughter did Disney last year, and we've all done several Carnivals, Princess, and HALs. Disney probably has the best kids activities, but my kids like Carnival and Princess fine too. I think the Disney stuff has more hands on activities that parents are supposed to participate in.
We just got back a couple weeks ago from Carnival Holiday and my daughter loved it. She's almost nine though. She was in the 6 to 8 age group. My son was in the 2 to 5 year olds and they did do activities but was probably more like daycare than camp. They did paint his face several times and they made Dora backpacks and he did have fun but not as much as my daughter did. She wanted to be there ALL the time! So, you might have better luck with it when he/she turns 6!
All ships are different though...our ship was the smallest so the 2 to 5's might be better on the bigger ships???